Gas-filled photo-electric cell



Al 1931- N. R. CAMPBELL 1,800,337

GAS FILLED PHOTOELECTRIC CELL Filed Feb. 10, 1928 2 Shets-Shoot 1' v CUQQENT VOLTAGE Tug/enter, I

Qpril 14, 1931- N. R. CAMPBELL 1,300,337

' GAS FILLED PHOTOELECTRIC CELL mad Feb. 10. 1928 2 Sheets-Shoot 2 CA; A2 P f? to my. a

Patented Apr. 14, 1931 PATENT f orrlca v NORMAN" BOIBER'I' CAMPBELL, OE 'WA'IFOBD, ENGLAND easmnnn PHQTO-ELEGIBIO CELL Application filed February 10,.1928, Serial No. 253,280, and. in Great Britain February 25, 1927.

This invention relates to improvements in methods of usinggasfilled photoelectric cells.

' For certain purposes, for example picture tion of light, varying with tic in series with radio telegraphy, it is desirable to produce a cur-rent varying with relatively high irequenc'y, say 5000 cycles per second, by the aca considerably lower frequency. This purpose may achieved by utilizingthe oscillatory current .given by a photoelectric cell working on a normally unstable portion-of its characterisa stabilizer. The frequency of the oscillations depends on the current limited by the stabilizer, the capacity and the illumination of the cell. p

In the accompanying drawings Figure 1 is i a curve of the unstable characteristics plotted trodes,

voltage against current; Figure 2 shows characteristicsdiagrammatically plotted frequencies against current and Figure 3 shows a suitable circuit arrangement for applying the invention.

First the meaning of characteristic must known that the current through an illuminatedgasfilledcell increases regularly with the potential difl'erence between the elecso long as this potential diflerence does not exceed a certain maximum, called the sparking potential. When the sparking potential is reached, a sudden increase of current occurs and a glow develops in the cell; when the term unstable the glow has been started the current willagain vary regularly-with the potential difference, but in a difierentmanner. A cell has therefore two potential [characteristics each defining aerelation between current and potential, one refers to-the state in which there is no glow discharge and is known as the Townsend characteristic, the other refers to the state in which there is a glow discharge and is known as the glow characteristic. Both these characteristics are stable, the

' current increases with the potential. The

Townsend characteristic depends on the illumination of the cell; theglow characteristic is almost independent of it.

These two potential characteristics' are shown in Figure 1 of the accompanying drawings. The figure does not represent accu-' be explained. It is rately the characteristics for any particular cell, but illustrates only those features of the characteristics, common to all cells, to which attention is about to be drawn. The two, curves T T represent-the Townsend characteristics of the cell for a less and a greater illumination respectively; G represents the glow characteristic, which is almost independent of the illumination.

It will be seen that the currents on the glow characteristic G are all greater than any of the Townsend characteristic; there is an intermediate range of current (namely that be tween the line X and the line X or X ac-. cording as the cell receives the less or the greater illumination) which does not belong to either of these. A continuous. current wlthln this range cannot be 'made to flow through the cell.

, But if the circuit external to the cell contams a current limiter, a current of which the mean value hes in this range can be made to pass through the cell. Thus there may be placed in series with the cell a thermionic dlode and a source of'potential greater than the sum of the sparking potential of the cell and the potential required to saturate the thermionic current, the temperature of the cathode of the diode being such that thesat- .urated current through it lies within this diode will charge up the condenser formed 'by' the electrodes of the cell (together with any condenser placed in parallel with them) until the sparking potential of the cell is reached. A glow discharge-will then pass through the cell, carryingla current greater than that. which flows rough the diode.

The condenser will therefore discharge until the potential indicated by the point R in Figure 1 is reached, and the glow discharge can 5 the same purpose the p tic, that "is to say, the fre ation between the.

be no longer it; The 3 current through the cell will-then cease and the 'con-.

denser begin to'c'harge up once more.,

Exactlythe same conditions will obtain-if.

5 a sufficiently high ohmic resistance is used as a current limitenin place of the thermionic. diode.v Its resistance must be;'so great that .the potential drop across it is always large compared withthat across the cell, so that a the current flowing through it'is practically in series w1th"it, namely one that is large' compared with the sparking potential of the cell; Accordingly when a current limiter of thls'kll'ld. is placed in circuit and adjusted so I that the value of the current determined by it 2o lies between X and X (or X a mean current of this value will pass around the circuit, but the current through the cell will'be intermittentand consist of a 'series of dischar similar to the glow discharge separate by intervals in which little current f fl0ws. Corresponding to the mean value of, the current there will be amean value of the" potential across the tube; the-relation be tween this mean current andmean potential-- will define another characteristic, joining the Townsend characteristic to the glow characteristic; since the meanyvoltagedecreases as e the mean current increases,.this intermediate characteristic ma "be-called the unstable I 38 characteristic;

"s unstable-characteristic is shown in Figure '1- by the lines-11 ,11 corresponding to the lesser. and greater .illumi-' -nations; really its form'is muchmore com plicated than that shown, but its slope is 40 always opposite in sign to that of the glow and Townsend characteristicap The methods for using photoelectricr'ells known hitherto, depend upon the fact that the relation between potential: and currentvaries with illumination the range of the Townsend characteristic," and they can'be de- 1 scribedjsimply in terms oft-he variations of f. thatcharacteristic. with light; The unstable characteristic also varies with light'and can 0 therefore be used,,like the Townsend char-.

acteristic, forth'e detection and measurement of light; method of using it have actually been proposed; But'in the range of the-unstable characteristic, it is possible to use for frequency characterismean current through the circuit and the frequency ofthe intermittence of the current through the cell; for this frequency characteristic also varies with the illumination. The present invention, in its widest aspects, consists of a method of using a gas-filled pho-.

t electric cell for the detection or measure- F ment of light or the control of currents by light which depends on the change produced vised by a study of the unstable portion of the 'iting device inseries with the cell andif the ring when the frequency is low can be stopped frequency increases with the illumination;

by light in the unstable "man at the ire quency characteristic of t e cell.

Methods according to the present invention suitable in, various circumstances can. be defrequency-current characteristic of a photo- Qelectric celL' Figure 2 of the accompanying drawings shows examples of the characteristic diagrammatically, frequencies beigg measured along CY and currents along 0 It must be understood that the curvesshown' arenot'those of any particular photoelectric cell; they are not drawn to scale and donot represent details; they illustrate merely those broad features common to all cells, or at least to all cells of a very general'type, that sire important for the purposes of the invention. Curves l and 2 refer respectively to a smaller and a greater illumination. Each of them startsat small curre-ntswitha. portion 86 0A, or ()Agwhere the-frequency is zero; this portion correspondsto the stable-Townsend characteristic. Next there :is a portion'A i B or A B along whichthe frequencyis finite .andfincreases with the current. 1 This is the unstable; portion of the characteristic, which can be realized only if there is acurrent llmpotentialapplied across the cell and limiteris" greater than the sparking potential. "The frequency depends, of course, upon the capacity'of the circuit, and not only on the current and the illumination; but the capacity -is.'as.- sumed-to be constant; At B or Bgthe stable glowdischarge starts and the frequency falls to zero at C 'or- C andremains zero whenthe current is increased further. Ex rimentally, however, zero fr uency is indistinguishable from infinite requenc and, the

change at B or B can'be descri a sudden increase of the frequency to infinity .in. the direction C or,--'C' Thisgsecond alsoas method of description is, the more convenient for my purpose, and po'ssiblymore inaccord- I ance with the physical involved. I. shall therefore regard the course of the characteristic-asOA B iG' (where C' is atinfinity,') and not OA -B, Cg; v

The portion A; B or A- B alone relevant to this invention,-falls into 3 ranges. There is the low frequency range; from A, or A to N, over which illumination produces a decrease of frequency; here oscillations occurby increasing the illumination. In the neigh- 1m bourhood of N the frequency is almost independent of the, illumination, Over the high frequency 'range betwe'enN and B or 13,, the Y but again,'since increase of'the frequency to infinity means a stoppage of the oscillations, the oscillatidns at low illuminations can be stopped by increasing the illumination. According to the invention either the high fre-' l gauze.

. the spherical type,

quenc or the low frequency range can be used or the detection of lig These features are believed to be common to all gas-filled photoelectric cells of existing types. But thereare certain differences between the two main existing types that require attention. In one type, which will be called the anode is verysmall which usually compared with the cathode% h p 1 I t e vesse 11 consists of the inner wall the other type, which will be called the. plane type, the areas of cathode and anode are not very different, the anode is often a gauze, and for the purpose of this statement the area of the gauze is taken to be the area of the plane surface having the same circumference as the the high frequenc range.

The choice of t e range and of'the appropriate type of photoelectric cell will depend upon the object to be. attained. If the object is to stop, bymeans of light, oscillations with a frequency of the order of 5000 per second, then the higher frequency range must be used,

- preferably with a plane cell, because only in this range can a frequency of intermittence of the current be obtained. But'the light required to stop the oscillations relatively great; thus,'if the cathode consists of potassium sensitized by the discharge in hydrogen and if the aperture through which light enters the cell has an area of 5 sq. ems.

' the light from a 60 watt gas-filled incandescent lamp required.

If on the other'hand the object is to detect at the distance of 1 metre may be very feeble illumination then theflow frequency range should be used; 'for, as might be expected, the method is in general the more sensitive the smaller is the current passing through the cell. The current may then beadjusted so that. the frequency is'of the order of 1 per second, and light from the watt lamp at a distance of several hundredmeters' may be detected by the change in this frequency. Alternatively but less sensitively, the'current may be ad usted sothat the cell is at the point A of its characteristic and any increase in current sets up an intermittency 'or oscillation of the current. The incidence '00 increase in current which is necessary to reof illumination may then be detected by the store the cell to the state in which any .incre use of current again causes intermittency.

A circuit arrangement suitable for applying the invention is shown in'Figure 3. Here 1 the gas-filled photoelectric cell with the cathode 2 and the anode 3; connected with the thermionic current'limiter 4 having an anode 5 and filament 6; the filament is heated by a battery 7 in series with an adjustable rheostai 8 by means of which the current flowin through the current limiter can be varie To the current limiter 4 a battery 9 is connected giving a potential greater than the sparking potential of the photoelectric cell. This battery is also connected to one winding 10 of a transformer, the other'winding 11 of f which includes the means for detecting or utilizing the oscillating current. Ifthe apparatus is to be used merely for detecting or measuring light the winding 11 may be connected to a telephone with or without intermediate amplification means. "If it. is to be used for controlling electrical signals the winding 11 will be connected to the sig nalling device.

At 12 is shown a condenser connected across the photoelectric cell. Its presence is not always necessay, but it is sometimes useful for adjusting the frequenc ofthe oscillations. It is particularly use .when the lower frequeney range is'being used to detect very small illummations. When the cell is very near the point A or A', on its characteristic it will sometimes be found that a small condenser 12, with a capacity of the order of 100 micro-microfarads improve the stability of the oscillations andincrease the'sensitiveness of the device. In certainfcircumstances in this region the frequenhy may be amaximu'm fora particular capacity; the method I then be most sensitivewhen this capacity a is used.

As has previously been described, when an intermittent current is flowing'through the cell, the efiect of a change in the inten's-- ity of illumination of the cell is to produce a change in the frequency of the intermittence. "An-example will serve to make this clear. Suppose that the current through the filament-6 ofthe current limiter 4 has been so adjusted that an intermittent current-is flowingthrough the oscillatory circuit com-- he battery 9, the current'limiter 4,

prising land the transformer winding 10',

the cell the mean value of the intermittent current being re OP slig tly greater than that of A and the intensity spending to curvel. The ordinate through the point P. will intersect curve- 1' at Q and the frequency of the intermission will core resented on Figure 2 byan abscissa of illuminationbeing that oorre-'- respond to the value of theordinate at-the oint of intersection." Ifnowthe intensity of illumination be changed to that corresponding to curve 2, it will be seen that the frequency of. intermission will be reduced to. that corresponding to. (1,. If the current initially flowing through the cell 1 be sufliciently small, the change to the more intense v illumination reduces the frequency-oi: intermissionoff'the-current flowing to the cs cile lato acircuitaforesaidto zero'. i at'Iclaim is: a 1. Means for controlling electricalficurrents of relatively high frequency by means 1 of light varying with relatively low ire-- quency which con: rises an electrical circuit arrangement inclu in series a gas-filled' i photoelectric cell, a current limiter substan-' tially as described, and a source of potential gre'ater thanthe s arking potential of the. cell the amounto current which it allows to iiow through the said c'ellibeing such that 3 the-said cell will operate on some part of the unstable frequency-current characteristic.

2. An electrical circuit arrangement com.- prising in series a' "gas-filled photoelectric v {3611 V1; current limiter substantially as de- 1that,an:intemiittentcurrent flows throughhcom a" filled hotoelectrlc im gas p p it'er submntiallyas'des'cribed, a source of potential greater than the sparkin (potential of the cell and means capable o etectingchan s in the v a: frequency of the current, conn in series,

.the current through the cell being so controlled by the currentlimiter that an inter mittent current flows-through the cell'and' .the frequency of said current varies with the 1-40 'illnminationfalling uponthe celL' NORMA}; ROBERT CAMPBELL. 

